Transmission of time referenced radio waves

ABSTRACT

The invention relates to the radio-electrical transmission of accurate timing-mark signals. A space-diversity electrical connection is formed between a station having one single antenna and a station having two antennae and one of the stations is equipped with a receiver device which combines into one composite timing-mark signal the different repetitive timing-marks decoded and rectified from high-frequency signals which have traversed several spatial propagation paths. A common automatic gain control circuit weights the decoded and rectified timing marks according to a weighting which diminishes with the level of high frequency reception. Preferably the rectification of the timing marks is carried out non-linearly.

The invention refers to the radio-electrical transmission of accuratetiming-mark signals. It is aimed more particularly at a terrestrialtransmission beyond the horizon and even very much beyond the horizon.

Installations are already known for the radio-electrical transmission ofa signal forming a timing mark. In these a radio-electrical connectionis defined between two antennae spaced apart. A transmitter applies toone of the antennae a high-frequency electrical signal coded accordingto a repetitive timing mark--for example, short synchronoushigh-frequency pulses from a time base. Corresponding waves arepropagated between the two antennae. And a receiver coupled with thesecond antenna receives the high-frequency signals like the transmittedsignals and decodes from them the timing-mark.

For the majority of the time the radio-electrical connection isterrestrial, that is to say, the two antennae are located at twodifferent points on the surface of the earth (continents or oceans).

In order to transmit a very accurate timing-mark it is necessary toemploy a very wide frequency spectrum and consequently to operate athigh-frequency (VHF or UHF, for example). Known installations of thisspecies operate well insofar as the distance between the two antennaeremains less than a limiting value, which depends particularly on theheight of the antennae above the surface and as one refers customarilyto the distance of the horizon seen from the transmitting antenna.

On the other hand for radio-electrical connections of a length greaterthan twice the distance of the horizon (for the majority of the timeabout 150 km), anomalies of propagation appear. These are manifested bya multiplicity of distinct times of propagation for the waves and by thereception of high-frequency signals of comparable amplitudes which maylie in phase opposition.

Under such conditions conventional timing-mark transmissioninstallations cannot operate correctly for reasons which will beconsidered in detail later.

The practice is furthermore known of employing long-distanceradio-electrical connections in telecommunications because the transittime is of little importance in this type of application. One currentsolution consists in providing a radio-electrical connection withspace-diversity, that is to say, it includes a number of antennae at oneend of it at least, these being antennae which define a number ofdifferent global propagation paths. At the receiver this solution makesuse of both or either of the following arrangements:

switching of the antennae in order to retain only that which gives asuitable signal at a given instant;

coherent summing of the high-frequency signals received at the variousantennae or of intermediate frequency signals which are deduced fromthem.

The expression "coherent summing" signifies that the phase of each ofthe signals is shifted in order to bring them to the same phase athigh-frequency (phase subjection) and only to add them afterwards.

These conventional arrangements of transhorizon telecommunicationconnections have not been able to be employed when it is a question oftransmitting a timing-mark. In short they do not enable a sole andstable propagation time to be defined to which the receiver can besubjected.

The present invention offers installations for radio-electricaltransmission of signals forming a timing-mark, these being installationswhich operate beyond and even very much beyond the horizon, for example,as far as 3 to 10 times the distance of the horizon. The installationsof the invention make use of a space-diversity radio-electricalconnection but in a different way from that which has been employedhitherto in telecommunications.

The basic structure of the installation of the invention is the same asthat of conventional installations for the transmission of atiming-mark: the installation comprises a first and a second antennastation spaced apart and capable of cooperating by wave means in orderto define a radio-electrical connection; a transmitter device coupled tothe first antenna station is capable of applying to it a high-frequencyelectrical signal coded according to a repetitive timing-mark in orderto transmit corresponding waves; and a receiver device coupled to thesecond antenna station is capable of receiving the high-frequencysignals like the signal transmitted as well as of decoding from them therepetitive timing-mark.

In accordance with the invention the radio-electrical connection is ofspace-diversity type: one of the antenna stations comprises at least twoantennae spaced apart in order to define for the waves at least twodifferent propagation paths. And the receiver device combines into onecomposite timing-mark signal the different repetitive timing-marksdecoded and rectified from the high-frequency signals which havetraversed the several propagation paths.

The difference from transhorizon telecommunication installations isevident: the composite signal is obtained after decoding and rectifyingby a non-coherent rearrangement, taking no account of the phase of thehigh-frequency signals received. One no longer tries by switching ofantennae to select one high-frequency signal received amongst many; onthe contrary, the different decoded timing-marks are systematicallycombined.

The receiver device very advantageously weights the decoded timing-marksaccording to a weight which diminishes with the level of high-frequencyreception over the propagation path from which they arise.

For the majority of the time in practice the receiver device includes acircuit for variable-gain selective amplification of the incidenthigh-frequency signals. In accordance with the invention the variablegain is defined according to the crest amplitude of the decoded andrectified timing-marks. The weighting in accordance with the inventionis realised by the fact that the receiver device defines a commonvariable gain for all of the high-frequency signals received,independently of their propagation paths, from the crest amplitude ofthe composite timing-mark signal. The result is a weighting of thedecoded timing-marks as a function of the level of the high frequencysignal from which they have arisen, hence according to the propagationpath of the wave which has produced each high-frequency signal.

The difference with respect to transhorizon telecommunicationinstallations is clear: instead of sorting out the signals receivedaccording to their amplitude and/or their phase at high frequency,reference is made to the crest amplitude of the composite signalobtained after decoding and rectifying. By doing this one takes intoaccount the timing-mark of greatest amplitude--after weighting anddecoding--amongst the distinct different timing-marks which thecomposite signal possibly contains.

The variable-gain control applied in common to the high-frequencysignals received advantageously includes a predetermined time constantchosen to be greater than the average time interval for autocorrelationof the anomalies observable in transhorizon propagation.

In accordance with another important characteristic of the invention thereceiver device includes downstream of the selective amplificationcircuit a non-linear member which is usually the rectifier. This memberapplies a non-linear characteristic to the timing-mark signals so as toreduce again the weight attached to those which correspond with lowamplitudes of the high-frequency signal. With installations inaccordance with the invention the Applicants have obtained overtranshorizon connections much better results than with installations ofthe former technology. Thus installations in accordance with theinvention working with decimetric waves have operated as far as maximumdistances lying between three and ten times the distance of the horizon.

These good results which are still not completely explained seem largelydue to the following two facts observed by the Applicants: thecombination of the shortest transhorizon propagation path with that orthose which result from important secondary responses--heavy refractionin particular--leads to disturbances of amplitude and propagation timewhich are strongly correlated between them; again, a very rapid spatialdecorrelation of the waves received is apparent especially when theeffect of the disturbances is a maximum--phase opposition (on thetheoretical plane this corresponds with a random deformation of the waveplane).

Furthermore timing-mark signals can now be transmitted with ahigh-frequency energy sufficiently large to enable their reception verymuch beyond the horizon in spite of the considerable attenuation whichis then undergone by the waves.

It is very advantageous for this purpose to employ a coding of thetiming mark which enables the energy transmitted to be distributed intime, in particular a pulse compression code or better still apseudo-random code.

Thus the invention enables a timing-mark transmission which is bothtrustworthy (measurement possible under all circumstances), reproducibleon long term, and stable on short term. This is obviously fundamental inmeasuring time of propagation or distance.

Other characteristics and advantages of the invention will becomeapparent from reading the detailed description which is to follow, madewith reference to the drawings attached in which:

FIG. 1 illustrates a timing-mark transmission installation in accordancewith the former technique;

FIG. 2 illustrates the wave forms at different points of theinstallation as FIG. 1;

FIG. 3 illustrates a normal path and a disturbed path between atransmitter and a receiver;

FIG. 4 illustrates a first embodiment of an installation in accordancewith the invention;

FIG. 5 illustrates a space-diversity connection between a transmissionantenna and two reception antennae, with a normal path onto one antennaeand a disturbed path onto the other;

FIG. 6 illustrates wave forms taken at different points of theinstallation as FIG. 4;

FIG. 7 illustrates a variant upon the space-diversity by switchingantennae in the receiver of the installation;

FIG. 8 illustrates wave forms taken at different points in theinstallation as FIG. 7;

FIG. 9 illustrates another variant upon the space-diversity connectionby switching antennae in the transmitter of the installation;

FIG. 10 illustrates wave forms taken at different points in the circuitas in FIG. 9 in the case of a timing-mark coding by short high-frequencypulses;

FIG. 11 illustrates wave forms taken at different points in the circuitas FIG. 9 in the case where the timing-mark is transmitted by apulse-compression code;

FIG. 12 illustrates an embodiment of the invention in which thespace-diversity is of the type as FIG. 9 and in which the timing-mark isrepresented by a pseudo-random code;

FIG. 13 illustrates wave forms relative to the embodiment as FIG. 12;

FIG. 14 illustrates an application of the invention to the measurementof the distance between an interrogator and a responder;

FIG. 15 illustrates an application of the invention to the measurementof distances between an interrogator and a number of responder(determination of position in circular radio-navigation); and

FIG. 16 illustrates an application of the invention to the measurementof the distance between a movable receiver and fixed stations comprisinga transmitter and a number of responder (determination of position, forexample, by radio-navigation in the hyperbolic mode).

I--FORMER TECHNIQUE

In order to define better the technical field of the present inventionan example will first of all be described by reference to FIGS. 1 to 3of an installation according to a former technique.

A first antenna station 101 and a second antenna station 201 areequipped each with one single antenna. They are spaced apart and cancooperate by wave means in order to define a radio-electricalconnection.

A transmitter device coupled to the antenna 101 includes a transmissionamplifier 111 preceded by a coding circuit 112 which in turn receivesthe signals from a first time base 113. The time base 113 transmitspulses of period T_(R). If T₁ designates the instant of appearance ofone of the pulses the others will be separated from it by a whole numberK, positive or negative, of periods T_(R). Thus the time base 113defines a repetitive timing-mark which will be written T₁ +K.T_(R).

The transmission coding circuit 112 has the duty of cooperating with thetransmission amplifier 111 in order to apply to the antenna 101 ahigh-frequency electrical signal coded according to the repetitivetiming-mark T₁ +K.T_(R). In FIG. 1 the coding consists in making apowerful and short pulse of purely high-frequency correspond with eachpulse supplied by the time base 113 or to only certain of them. FIG. 2ashows the pulses transmitted (here and in what follows, thehigh-frequency included in the pulses is not represented in order tosimplify the drawing).

The high-frequency signal which carries the timing-mark is transformedby the antenna 101 into waves which are propagated through a propagationmedium towards the antenna 201. Because of the time of propagation ofthe waves a delay T_(p) is found to be applied to the timing-mark whichthey carry, when they arrive at the antenna 201.

The antenna 201 is coupled to a receiver device which comprises first ofall a selective receiving amplifier 211 which passes a band like thatwhich has been chosen for the transmission. The output from theamplifier 211 supplies high-frequency pulses delayed by T_(p) withrespect to the pulses transmitted (FIG. 2b). The amplifier 211 isfollowed by a decoder circuit 212 consisting here of a simple switch(for the case of coding by short pulses), followed in turn by a detectorcircuit 213 comprising, for example, a rectifier diode.

Furthermore in the receiver a time base 251 defines recurrent pulseswritten T₂ +P.T_(R). Their period T_(R) is the same as at thetransmitter (FIG. 2c). But the other characteristics T₂ and P of thetiming-mark so defined at the receiver have no reason for beingidentical with those of the transmitter.

A controlled delay circuit 252 produced different forms of pulsesshifted in phase by the same variable delay as the pulses of the timebase 251. An output 2521 from this circuit supplies pulses of the sameduration as the transmitted high-frequency pulses. FIG. 2d illustratesthe form of these pulses on the time scale of FIGS. 2a to 2c. FIGS. 2e,2f and 2h give enlarged representations corresponding with the outputsfrom the circuit 252, while FIG. 2g gives an enlarged representationcorresponding with the output from the detector 213.

The decoder 212 is here a simple switch which carries out a correlationbetween the high-frequency pulse received (FIG. 2b) and the output 2521from the delay circuit 252. Insofar as the delay is suitably adjustedthe correlation is total, the two signals coincide and hence the decoderlets pass the whole of the high-frequency pulse received and suppressesthe noise present between the pulses. Otherwise a portion of the highfrequency pulse received is removed because the closure of the switch212 does not coincide exactly with this pulse. After that the detector213 rectifies the whole or the remaining portion of the high-frequencypulse received, as the case may be.

Initially in a pick-up phase the delay in the circuit 252 is made tovary rapidly until a signal is obtained at the output from the detector213. After that the delay is adjusted finely.

For this purpose the output from the detector 213 (FIG. 2g) is appliedof one input to a time discriminater 254. In one embodiment the latterreceives as its other input a bipolar signal supplied by the output 2522from the circuit 252. This bipolar signal (FIG. 2f) has a negativeportion followed by a positive portion, symmetrical with one another.The transition between them coincides with the centre of the signalgiven by the output 2521. The time discriminator multiplies the outputfrom the detector 213 by the bipolar signal 2522. Hence the output fromthe time discriminator will be positive or negative according to whetherthe rectified pulse leaving the detector 213 is late or early withrespect to the output from the delay circuit 252. It will be zero in thecase of exact coincidence.

The negative or positive signals thus obtained are applied to anintegrating filter circuit 256 which controls the delay in the circuit252 and forms with it a delay correction loop. The circuit 256comprises, for example, two consecutive stages of filtration in order toachieve a second order filtration, one of these filters (often thefirst) being corrected so as to avoid instabilities in the delaycorrection loop. In that way the delay in the circuit 252 is permanentlycorrected as a function of the high-frequency pulses received. Theresult is at the output from the delay circuit (output 2523, FIG. 2h) atiming mark synchronous with that from the transmitter but shifted inphase by the propagation time T_(p) and written T₁ +T_(p) +K.T_(R).

For the majority of the applications a clock circuit 260 is connectedbetween the output from the time base 251 and that from the controlleddelay circuit 252. The separation in time measured by the clock 260 isgiven by the expression:

    T.sub.1 +T.sub.p -T.sub.2 +(K-P).T.sub.R.

In this expression the period T_(R) is known. K and P are a priori anywhole numbers but it is in general known how to determine the differencebetween them (methods of removing ambiguity, for example).

In measuring distance one often knows T₁ -T₂, and one is looking for thepropagation time T_(p) in order to measure the distance from transmitterto receiver. A number of measurements made between more than two pointsfurthermore enables the distance to be determined by eliminating theterm T₁ -T₂ if it is unknown.

In a time measurement application which can be envisaged as the mainpurpose (T_(p) known) or in complement to the foregoing, the separationT₁ -T₂ between the two timing-marks is measured in order to haveavailable the same time reference at the transmitter and the receiver.

In an installation such as that in FIG. 1 the amplifier 211 is ingeneral equipped with an automatic gain control. It may be seen that theoutput from the detector circuit 213 consists of rectified pulses whichalready form a timing mark signal. The amplitude of these pulses isproportional to the amplitude of the high-frequency signal present atthe output from the receiving amplifier 211. Hence the output from thedetector circuit 213 is the input signal to the automatic gain controlchain which starts with an integrating filter circuit 203 the duty ofwhich is to determine the amplitude of the pulses. This integratingfilter circuit 203 is followed by an automatic gain control circuit 204which controls the gain of the receiving amplifier 211 so that theamplitude of the pulses detected remains in the vicinity of a referencelevel chosen for optimum reception of the strongest high-frequencypulses.

Such a timing-mark transmission installation has a limited "separatingpower" the fineness of which is proportional to the width of the band oftransmission and reception. In order to transmit a timing-mark suitablyit is first of all necessary that the receiver be able to determine apropagation time which is stable and unique at least within itsseparating power, and of course that the useful high-frequency signalwhich it receives can be distinguished from the noise.

The achievement of these two imperatives is largely dependent upon themodes of propagation of the waves. The wave propagation is generally thesame for frequencies higher than 30 MHz and the analysis which is madeof it below is applied by way of example to metric, decimetric andcentimetric waves (30 MHz to 30,000 MHz).

In the field of propagation distances running up to 1.5 to 2 times thedistance from the transmission antenna to its geographical horizon, orabout 100 km, installations of the former technique in general operatewell. In this first field one distinguishes the zone located on thisside of the horizon where the propagation is effected in line of sight,then a zone located up to 1.2 times the distance of the horizon wherethe propagation is due to the normal refraction of the waves by theatmosphere and finally a diffraction zone or "shadow" zone where thepropagation offers substantially the same characteristics (FIG. 3,path 1) but where the level of the waves received is strongly attenuatedwith distance.

In this first field the ground can be reflection produce interferencebut the differences in path which result from it are small and do notcause important scattering of the propagation time of the waves toappear (it will be recalled that the propagation time results from thecombination of an infinity of elemental paths, whence the possibility ofits scattering).

Casual reflections (especially reflections in the vicinity of theantennae) may produce a secondary propagation path whence a secondarypropagation time results, differing from the normal time. But that israther rare and the separating power of the receiver is often sufficientto eliminate the secondary path. Hence users can in general put up withthe short times of unavailability of the installation which result fromit.

Beyond twice the distance to the horizon starts a second field where theinstallations of the former technique often operate less well becausethis second field associates a basic propagation ("troposphericdiffusion") which is still usable, with abnormal and discontinuouspropagation effects (heavy tropospheric refraction or "superrefraction") which may be frequent in certain regions and in certainweather.

Beyond twice the distance to the horizon the tropospheric diffusion(FIG. 3, path 1) takes the lead over the effect of diffraction. This isexhibited as a level of the waves received which is subject to rapidfluctuations and relatively stable in the long term. However, thepropagation time undergoes a random scattering, substantially Gaussian,which increases with distance. The conventional installations can stilloperate under these conditions, contingent upon suitable filtration ofthe noise associated with the propagation time. But on the other hand aloss appears, on the average, of the energy which can be employed foracquiring the propagation time corresponding with the path with whichthe time tracking loop is in step.

As soon as they are added to the tropospheric diffusion the abnormaleffects (FIG. 3, path 2) define a second propagation time, in generallonger, and cause a considerable supplementary contribution to theamplitude of the waves received, which is called a "secondary response".

The conventional installations are generally overflowed by thesesecondary responses; very often their separating power does not allowthem to distinguish the main response from the secondary response inorder to eliminate the latter. And very frequently the two responsesarrive in phase opposition (as regards the high frequency signal); thereceiver then "sees" a grossly erroneous propagation time, where theerror may be bigger than the separation between the propagation times ofthe two responses. (This phenomenon tends to be further aggravated bythe receiving automatic gain control). Again, if the disturbance ispersistent the propagation time tracking device will fall out of stepwith the normal propagation time and subsequently lose a substantialtime in finding it again.

Hence in the face of these difficulties the conventional installationsdo not give absolute satisfaction.

On the contrary the installations in accordance with the invention givemuch better results in the presence of secondary responses (multiplepaths), whether it is a question of tropospheric refraction at longdistance or even of casual reflections at shorter distance. Moreoverthey function better in the zone of tropospheric diffusion even in theabsence of disturbances.

II--FIRST EMBODIMENT OF THE INVENTION

FIG. 4 illustrates a first embodiment of the invention. It includesnumerous parts like those in FIG. 1, which keep the same referencenumber and will not be described again.

In accordance with the invention the receiver includes a second antenna202 separated from the first by about 15 (201) to 30 wavelengths of thecarrier high-frequency. With this second antenna 202 is associated asecond reception channel which includes like the first a selectivereceiving amplifier circuit 221, a decoder circuit 222, and a rectifiercircuit 223. And the outputs from the two rectifier circuits 213 and 223are joined, thus combining the signals received over the two apparentpaths defined between the transmission antenna 101 and the two receiverantennae 201 and 202 respectively.

The composite timing-mark signal thus obtained is applied to apropagation time tracking loop generally like that in FIG. 1 (parts 251,252, 254 and 256). This tracking loop will be realised in practice bymeans of analogue integrators or counting-scale integrators, analogue ornumerical dephasers, oscillators controlled in frequency or in phasewith the analogue or numerical signal, or any equivalent means,especially numerical calculation algorithms equivalent to the wirednumerical chain comprising integrators, a dephaser, and an oscillator ortime base.

The respective connections between the antennae 201,202 and theamplifiers 211, 221 must be of the same length so as to present the samedelay time (possibly to the nearest whole multiple of the period of thetiming-mark). Similarly the delay times in the amplifiers 211, 221 andthe decoders 212, 222 must be the same.

The gains of the two receiving amplifiers 211 and 221 are veryadvantageously controlled in an identical way by a single automatic gaincontrol chain--integrating filter circuit 203 and gain control circuit204--connected to the common output from the detector circuits 213 and223 in order to receive from them the composite timing-mark signal. Theintegrating filter circuit 203 determines the crest value of thecomposite signal and it stores it with a time constant which isnecessarily at least equal to the period T_(R) of the time base. Thistime constant (for the increase of the gain) is moreover chosen to begreater than the average time interval of autocorrelation of thedisturbances of level due to the tropospheric refraction. In practicethe Applicants have found that this time constant should be greater thanabout 15 seconds.

The result is in general that the decoded timing-marks become weightedas a function of the level of the high-frequency signal from which theyhave arisen.

Reference will now be made to FIG. 5 which illustrates an example ofconditions of propagation between the transmission antenna 101 and thereceiving antennae 201 and 202.

In this example, which corresponds with a frequently occurringsituation, the antenna 201 receives only waves which have undergone anormal diffraction and/or diffusion; the antenna 202 receives the samewaves increased by a secondary response due to tropospheric refraction(or to refraction over an obstacle). And the combination of the two inphase opposition is shown by a collapse of the high-frequency signal atthe output from the amplifier 221. That is illustrated in FIG. 6 in thecase of time coding by simple high-frequency pulses. The pulsestransmitted are shown diagrammatically in FIG. 6a, and FIGS. 6b and 6cshow respectively the outputs from the amplifiers 211 and 221: it isseen that the output signal from 221 is 30 times less than that from theamplifier 211 (although the scale of FIG. 6 does not enable it to beshown, the two output signals are shifted in time). After the decodingswitches have eliminated the noise the combined output from thedetectors 213 and 223 joins the two signals into a single timing-marksignal consisting essentially of the stronger of them (FIG. 6d).

In accordance with an important aspect of the invention a non-linearrearrangement is thus achieved of the timing-marks received: in fact thediodes incorporated in the detectors 213 and 223 have a threshold andnon-linear response which contributes greatly to favouring the strongersignals. In practice it will be arranged that the weak signals (forexample, 1/30 of the strong signal) are completely eliminated by thenon-linear element incorporated in the detectors 213 and 223. This maybe achieved for certain by putting after the detector circuit animpedance separator or isolator and rearranging the outputs from the twodetector circuits by means of an "analogue OR" circuit enabling thesignal the level of which is higher to be practically the only oneperceivable at the output.

The rearrangement of the timing-marks received is likewise non-coherent,since it does not take into account the phase of the high-frequencysignals received.

The composite signal obtained after this non-linear and non-coherentrearrangement is employed as the input to the automatic gain controlchain. Consequently the relative proportions of the two signals received(FIGS. 6b and 6c) will not be modified by the automatic gain controlsince only the stronger of them is taken into account for regulating thegain.

And if it happens that the two signals are disturbed at the same time,the automatic gain control will wait before increasing the gain (becausethe time constant is longer than the average interval of time forauto-correlation of the disturbances of level).

Thus a weighting of the timing marks is preserved as a function of theamplitudes of the high-frequency signals from which they have arisen.

Whereas it has proved necessary to make a non-linear rearrangement ofthe timing-marks received for the automatic gain control the Applicantshave observed that one can employ a linear (but not coherent)rearrangement of the timing-marks received for feeding the time trackingloop. Such a linear rearrangement corresponds with pure rectification.Nevertheless it is at present considered preferable to employ also forthe time loop a non-linear rearrangement such as that which is allowedby the characteristics of the diodes or by an analogue-OR circuit. Infact such a non-linear rearrangement diminishes further the weightassigned to the timing-marks received over a low level of high-frequencysignal.

Thus in FIG. 4 it is the common output (non-linear) from the detectorcircuits 213 and 223 which serves as input both to the automatic gaincontrol chain and to the time tracking loop.

The advantages of the arrangement in accordance with the invention maybe explained as follows:

(1) When normal refraction does not exist in the tropospheric diffusionzone the signals which reach the receiving antennae are the object of afluctuation in amplitude, of a scattering of the propagation time, aswell as of a fluctuation in the angles of incidence of the waves whichcompose them. A deformation likewise appears of the "plane" forming the"wave front" and consequently a rapid spatial decorrelation. Thus thefluctuations in amplitude and in propagation time observed in thesignals collected by the two antennae 201 and 202 (arranged along analignment perpendicular to the main direction of the incident waves andat a distance apart of 15 to 30 wavelengths) are in practice only veryweakly correlated.

By rearranging the timing-marks detected over the two channels thefluctuation of the composite signal is weaker than that which would beobserved over each channel independently because of this decorrelation.It will be observed that here a linear summation is just as suitable asa non-linear rearrangement.

(2) When a refraction or an abnormal "super-refraction" exists. In thiscase as has been explained previously, secondary responses may appearfrom the propagation medium. These secondary responses develop ratherslowly in time and in space and their time of arrival is slightlyshifted with respect to the "useful" or normal response from the mediumand consisting of the signals proceeding from the normal troposphericdiffusion. The two responses may interfere at a given place with similaramplitudes and any possible phase combination.

When the two responses are combined in phase agreement and up to ±2 to2.2 radians, the disturbances in amplitude and in apparent propagationtime are weak. The device in accordance with the invention then operatesin practice as in the absence of these disturbances (Case 1 above).

When on the contrary the main response from normal troposphericdiffusion has added to it a secondary response nearly in phaseopposition (at about ±0.1 radians) the composite amplitude of the wavereceived decreases considerably and considerable fluctuations appearcorrelatively in the apparent propagation time associated with the wavethus received.

Under these conditions the device in accordance with the inventionoperates in the manner described in relation to FIG. 6 by retaining onlythe strongest signal for the automatic gain control. Thus one is sure ofpreserving the relative amplitudes of the high-frequency signalsreceived. In order to feed the propagation time tracking loop thesignals leaving the two channels are rearranged, weighted because of thecommon gain control.

In this second case the rearrangement may be linear but it is moreadvantageous that it be non-linear so as to diminish still more theweight associated with the weak signals.

FIG. 6 illustrates the situation at a given instant when it is theantenna 201 which is receiving the normal tropospheric diffusion signalwhilst the antenna 202 is receiving the signal disturbed by a secondaryresponse from tropospheric refraction, lying in phase opposition. Inpractice the situation develops permanently and may very well appear inthe reverse manner.

The advantages of the device of the invention are considerable: when anabnormal refraction exists, the probability of seeing a disturbedchannel and a conventional receiver caught out, may be of the order of1/20 to 1/30 of the time; as the appearance of disturbances between thetwo channels is relatively little correlated, the probability of findingdisturbances simultaneously on the two channels, that is to say, ofobtaining poor operation of the device of the invention is about 1/400.

It has been indicated above how to determine the time constant forincrease in gain in the automatic gain control chain. It is clear thatthe choice of this time constant contributes to the obtaining of goodresults with the device of the present invention. Obviously the sameapplies as far as the choice is concerned of the zone of non-linearityof the diode or of the analogue-OR circuit incorporated in the detector,with respect to the range of amplitude available as output from thedecoder. It is in fact this zone of non-linearity and its possiblethreshold which will complete the weakening of the weighting carried outupon the signals of weak amplitude.

III--VARIANTS UPON TRANSMISSION OF THE TIMING-MARK

The greater the power transmitted, the better will the installations inaccordance with the invention operate for enabling the receiver todistinguish the useful signal from the noise in spite of the weakeningundergone by waves over long distances.

Furthermore in order to transmit a timing-mark one may, for example,produce short synchronous high-frequency pulses from a time base, whichcorresponds with a total amplitude modulation of the high frequency oftransmission from pulses deduced from the time base.

More generally, to produce a high-frequency signal coded according to atiming-mark always necessitates a modification or modulation of the purehigh-frequency wave shape. In fact as soon as a modulation has beencarried out upon the latter, of known shape in time and of the sameperiod as the time base 113, the signal thus modulated is the bearer ofa repetitive timing-mark connected with this time base. Any signal ofthis species may be employed according to the present invention.

A known example of a code of this species is the code known as a "pulsecompression" code. Usually it makes a linear excursion from the highfrequency of transmission into a known frequency interval and for aknown interval of time, correspond with a time base pulse. Upondecoding, a convolution filter ("matched filter") enables the reversetransformation of the signal received into time base pulses. Fordecoding one may likewise but less advantageously correlate the codereceived with a local replica of same code.

In practice the periodic modulation which represents the timing-mark isoften defined by a predetermined periodic sequence of low and highlogical levels. The period of the sequence is of the time base and thestart of this sequence is synchronized with the time base pulses. Such asequence may conveniently be represented by a binary number and anassociated clock frequency. Furthermore such codings by binary sequenceare usually accompanied by a frequency or phase modulation.

A particularly advantageous example of binary sequence codes is that ofthe "pseudo-random codes". In a manner known in itself such codes areobtained by means of a shift register, to the input to which a logicalcombination is brought back from at least two of its stages. Undercertain conditions such a register provides within a finite interval oftime a practically random sequence of binary digits called"pseudo-random sequence or code". With such a pseudo-random code themodulation to the transmission which is employed is very often amodulation by phase reversal. Although decoding may be effected byconvolution it is more advantageous in certain applications to effect itby correlation.

A pseudo-random code is generally chosen, the duration of which is inrelation to the period of the time base to be transmitted and inprinciple at most equal to the latter. When the number of binaryelements in the code is increased at constant duration of it, itsautocorrelation function becomes stronger and more selective and thetransmission of the timing mark is consequently more accurate.

Although the invention is applicable whatever the mode of transmissionof the timing-mark it develops its advantages much better with codeswhich enable a spreading in time of the energy transmitted, this havingto be large in order to enable good reception at long distances. In thisrespect it will be observed that the binary sequence codes of durationequal to the period of the time base enable an optimum spreading of theenergy transmitted; instead of short high-frequency pulses the energy isthen transmitted continuously.

As regards now the accuracy of transmission of the timing-mark, the moreselective the decoding is in the time, the better it will be, which hasproved important for distinguishing in the best way the main responsefrom the secondary responses. In this respect the Applicants at presentprefer the following associations:

coding by pulse compression code and decoding by convolution, especiallywhen a number of a priori unknown propagation distances must besupervised at the same time (the case of a radar incorporating both thetransmitter and the receiver, and supervising an indefinite number ofmoving objects reflecting the waves, for example);

coding by pseudo-random code and decoding by correlation with a replicaof the same code, especially when an a priori limited number ofpropagation distances is being tracked in a subordinate manner (thecase, for example, of movable objects equipped with receivers forcircular or hyperbolic radio-navigation, cooperating with two or threetransmitter stations).

The foregoing considerations of the coding of a timing-mark areapplicable to any of the described embodiments of the invention.

With certain methods of decoding-convolution in particular--it is notnecessary to provide a connection between the controlled delay circuit252 and the decoding circuit or circuits 212 (and 222, in case of need).That is why this connection is shown in dash-dot line on the electricaldiagrams, starting from FIG. 4.

IV--APPLICATIONS OF THE INVENTIONS

Various types of application of the invention will now be examined:

(a) the time of transit of the waves is known by having been measuredpreviously and/or because the distance from the transmitter to thereceiver is known; the timing-mark from the receiver can then besynchronized with that from the transmitter.

(b) the time bases of the transmitter and the receiver are alreadysynchronized, for example, because they are at the same place and have acommon time base; one can then have access to the propagation distanceby measurement of the propagation time. This is the case of the radartransmitter-receiver, the propagation path out and back of which isdefined by a reflection from a fixed or movable object. It is also thecase of a transmitter-receiver known as an "interrogator" working with areceiver-transmitter or "responder" which contents itself withretransmitting the timing-mark after shifting it by a whole number ofperiods T_(R) (active circular radio-navigation).

(c) a main transmitter is employed and at least two fixed retransmitterslike the responders above; the relative positions of these threestations are known or can be determined; a movable receiver having threereception channels associated respectively with the main transmitter andthe two retransmitters can determine its position in the form ofdifference in distance with respect to the three stations by way of thedifferences in transit time (hyperbolic radio-navigation).

(d) As a corollary in Case (c), since there is available enough timinginformation for determining the position of the movable receiver, thetransit time parameter may be eliminated and the timing mark from themovable receiver may be synchronized with that which is common to thetransmitters.

Other applications exist, especially variants upon those above. Insteadof differences in distance alluded to in (c) one may employ summationsof distances or more generally linear combinations of distances.Furthermore without the various time bases employed being strictlysynchronous they may be very stable (very good quartz or atomic clocks,for example); in cases (b) and (c) "pseudo-distances" are then measuredwhich may be corrected after measurement of the deviations of thevarious time bases with respect to one of them. With time bases ofaverage stability pseudo-distances may still be obtained which will becorrected according to the indications from correction stations equippedlike the movable objects and transmitting corrections permanently or atsufficiently close intervals of time, taking into account the stabilityof the time bases (differential radio-navigation--circular orhyperbolic, for example).

Other embodiments of the invention will now be described which differfrom the first (FIG. 4) either by the method of employment of spacediversity (FIGS. 7 and 9) or by the method of coding of the timing-mark(FIGS. 11, 12 and 13).

V--VARIANTS AS TO SPACE-DIVERSITY

FIGS. 7 and 8 illustrate a variant upon the first embodiment of theinvention (FIGS. 4 to 6) in which instead of employing two separatereception channels between the two antennae 201 and 202 and the input tothe tracking loop (circuit 254) a single channel is employed, connectedin shared time to the two antennae.

For this purpose the output from the timebase 251 (FIG. 8b) is connectedto a circuit 206 for dividing by two, the output signal from which (FIG.8c) controls a switch 205. The latter alternates between two positions Aand B in which it connects respectively the antennae 201 and 202 to theinput to the single reception channel (211 to 213). The lengths of theconnections between the antennae 201 and 202 and the switch 205 must beidentical.

The high-frequency output from the transmitter is represented in FIG.8a. It is assumed as indicated in FIG. 5 that the channel A (antenna201) receives an undisturbed signal (FIG. 8d) whilst that from channel B(antenna 202) is disturbed, for example, by a tropospheric refraction inphase opposition (FIG. 8e). As the output from the amplifier 211 asignal is obtained which is formed alternately from the channel A andfrom the channel B (FIG. 8f). As the output from the detector 213,because of its non-linear characteristic, only the timing-mark signalspersist which arise from the channel A.

In set-off against the alternate use of the same reception channel, thetiming-mark signal is available only every other time when one of theantennae receives a disturbed signal. One skilled in the art knows thatthat is sufficient for making the time tracking loop function muchbetter than if a disturbed signal were preserved in it. Of course inthis variant having only one reception channel a gain control circuit(203,204) is connected between the output from the detector 213 and thevariable gain amplifier 211. As previously it is equipped with a timeconstant in order to take into consideration only the crest values ofthe timing-mark signal (FIG. 8g). The period of the latter is in thepresence of disturbances double that which it was in the case of FIG. 4;hence for the circuit 203 an integration time constant will be taken,which is at least equal to double the period of the time base.

The average rhythm of appearance of the timing-mark may be divided bytwo. Hence a time constant may be taken which is much larger thanpreviously but not necessarily double, for the integrating filtercircuits 256 of the tracking loop or at least for the first of them.

Furthermore if it happens that the output from the time base 251 isnearly in phase with the signals received, and output may be employedwhich is shifted by half-a-period of the time base in order to controlthe switch 205 through the divider 206.

In FIGS. 9 and 10 the space-diversity is achieved by two antennae 101and 102 located at the transmission end. They are connected alternatelyby connections of identical lengths, to a switch 105 which receives theoutput from the transmitter circuit 111 (FIG. 10a). This defines oneradio-electrical channel A through the antenna 101 and one channel Bthrough the antenna 102. The state of the switch 105 (FIG. 10b) ismanaged by a divider by 2 (or an even number 2n) which is connected toone output from the time base 113, this output lying slightly in frontof that which defines the transmission pulses.

The receiver will then receive alternately as they present themselves,the two radio-electrical channels defined by the transmitter. FIG. 10crepresents the output from the amplifier 211, still in the propagationcase defined by FIG. 5 where the channel A is transmitting well and thechannel B is disturbed (except that in FIG. 5 the locations of thetransmitter and receiver are reversed). And FIG. 10d illustrates theoutput from the detector 213 which it can be seen is like that of thefirst variant (FIG. 8g). Hence it is employed in the same way.

The receiver of FIG. 9 is the same as that of FIG. 7 with, however, onlyone antenna and omission of the components 205 and 206.

The considerations developed with respect to the first variant about thedifferent time constants remain valid, taking into account the fact thatthe alternate reception period from the radio-electrical channels A andB may be doubled or multiplied by 2n, depending upon the choice of thedivider 106 in the transmission.

As previously, the time constant is advantageously increased for theautomatic gain control loop, whereas the increase of the time constantsis not necessarily useful in the time tracking loop since thatdiminishes the maximum speed of tracking of the latter because ofdisturbances which are produced on the average only over 1/20 of thereception time.

This second variant embodiment exhibits a particular advantage; as thespace-diversity is realized at the transmission a large number of simplereceivers may profit from it (even up to a certain point, conventionalreceivers of the former technique).

In the space-diversity embodiments described only two antennae have beenemployed. Of course this number is not restrictive and three or more maybe employed. The increase in the number of antennae complicates thetransmitter and/or the receiver; in set-off, the probability that theinstallation is caught out by disturbances diminishes as the exponentialof the number of antennae.

VI--VARIANTS UPON THE CODING OF THE TIMING-MARK

In what has gone before, the very simple case has been considered of thecoding of a timing-mark by short synchronous high-frequency pulses froma time base. That being so, high energy must be transmitted in order toensure good transmission very much beyond the horizon in spite of theattenuation which is undergone by the waves there. Instead oftransmitting high energy in short high-frequency pulses it is more oftenpreferable to spread out this transmitted energy by employing a moreelaborate coding of the timing-mark. Such codings have been describedabove and are applicable to all of the embodiments of the invention.Certain examples of application of these elaborate codes will now bedescribed.

The first example of a code concerns the installation of FIG. 9 and thewave forms which it brings into play are illustrated in FIG. 11.

At each pulse of duration τ produced by the transmission time base 113(FIG. 11 A) the coder 112 makes a code start of duration τ' much greaterthan τ (FIG. 11b). In FIG. 11c is found again the alternate switching ofthe antennae 101 and 102 in order to define the radio-electricalchannels A and B respectively. These two antennae will radiatealternately portions of signal of high-frequency (not shown) which havea duration τ' (at least), and are modulated by the code of FIG. 11b. Thechannel B is assumed to be disturbed, whilst the channel A is not. Hencein the receiver as the output from the amplifier 211 the transmittedsignals are obtained, delayed by the propagation time T_(p), that is, atone time the contribution from the channel A of general amplitude S, atanother time the contribution from the channel B, of weaker generalamplitude, S/30, for example, because of two propagation paths arrivingin phase opposition at the antenna 201 of the receiver.

The code may be a priori of any kind and has not been illustrated indetail in FIG. 11. The codes at present preferred are a signal modulatedlinearly in frequency in a frequency interval equal to 1/τ or else asignal modulated in phase by a pseudo-random sequence.

In the receiver a decoder 212 can be employed which makes pulses ofduration τ (the same as at the transmission time base) correspond withthe coded signal received, with a signal-to-noise ratio muliplied by√τ'/τ (with reference to the signal-to-noise ratio of the high frequencyreceived). Such a decoder gives output pulses after a delay θ which isknown and which may be greater than the period T_(R) of the timing-mark,which does not trouble the operation of the time tracking loop.

FIG. 11e illustrates the output from such a decoder. After processing bythe tracking loop the signal of FIG. 11g is obtained as the output 2523from the circuit 252. For its part the output from the time base 251 inthe receiver is illustrated in FIG. 11f. And the clock 260 (FIG. 9) willnow provide a time separation increased by the constant delay θ due tothe decoder or T₁ +T_(p) +θ-T₂ +(K-P)T_(R) (FIG. 11g).

The description above, made with reference to FIGS. 9 and 11 correspondsexactly with the case of a pulse compression code at the transmissionand decoding by convolution at the receiver. In this case the decoderconsists of a filter matched to the particular code from thetransmission. Such a matched filter effects by construction aconvolution of the code, which provides a short pulse after a delay θ.For the decoding, the matched filter does not necessarily need to knowthe timing-mark onto which the time tracking loop is adjusted. Hence incertain cases the connection in dash-dot line between the circuits 252and 212 disappears. With this reservation, the general diagram remainsthat as FIG. 9. Of course with other types of space-diversityconnections the diagram of the installation would be that of FIG. 4 orof FIG. 7.

For the decoding, instead of carrying out a convolution one may proceedby correlation of the code received with a replica of the same codegenerated locally in the receiver with respect to the timing-mark uponwhich the tracking loop is adjusted. The principles developed above withrespect to the coding remain in general valid but the diagram forrealization often becomes more complicated, because it is preferable tooverlap the time discrimination function of the tracking loop and thecorrelation function of the decoder.

With reference to FIGS. 12 and 13, a particular embodiment will now bedescribed with decoding by correlation, for a coding consisting inmodulating high-frequency signal trains by phase reversals according toa pseudo-random sequence. The space-diversity connection is effected asin FIG. 9 (or as a variant as in FIGS. 4 or 7).

At the transmission end the clock 1130 now defines a frequency f=1/τ(13a) where τ is a sub-multiple of the period T_(R). This clock 1130feeds a pseudo-random code generator 1120 consisting of a shift registerof n stages, the input to which is fed by a logical combination from atleast two of its intermediate stages. In such cases τ must satisfy therelationship T_(R) =(2^(n) -1).τ. The logical combination in question orelse any one chosen in advance, of the stages of the register 1120 (FIG.13b) feeds a phase modulator 1121 which produces a signal intended forthe transmission and modulated in phase according to the output from thepseudo-random code generator (FIG. 13c). It will be assumed here that itis a question of modulation by phase reversal. Otherwise modulation maybe effected over a signal of intermediate frequency which a change infrequency then brings to the transmission frequency.

It has previously been seen that the clock 1130 operates at a period τwhich is a sub-multiple of that T_(R) of the time base. A divider offrequency by 2^(n) -1, designated by 1131, restores the time base T₁+K.T_(R) (FIG. 13d). As in FIG. 9 this feeds the remainder of thetransmitter device, in particular the switching of the antennae 101 and102 (FIG. 13e). In practice instead of a divider by 2^(n) -1, decodingof the content of the register 1120 may be employed.

At the receiving end as at the transmission end is found a clock 251 butof frequency 2/τ. The controlled delay circuit 252 now acts upon theoutput from this clock and in response to the signal delivered by theintegrating filter circuits 256.

The output from the delay circuit 252 is divided by two (in frequency),which restores the frequency 1/τ of the transmission, which is thenapplied to a pseudo-random code generator 2511 like that in thetransmission.

The output from the receiver amplifier 211 (FIG. 13f) is applied to adecoder circuit 212 consisting first of all of a modulator by phasereversal 2120 controlled by the code output from the code generator 2511(FIG. 13g), and then of a coherent integrating filter circuit 2121(output illustrated in FIG. 13h). The latter consists, for example, ofan L-C filter tuned to its input frequency, the time constant of whichis preferably at least equal to T_(R). In practice because of the wideseparation between the frequency of the carrier signal and that(1/T_(R)) of the timing-marks, one or more frequency changes areadvantageously employed before decoding (this is valid also with othertypes of coding, in particular with pulse compression codes).

As far as the automatic gain control is concerned there is found againas in FIG. 9 a rectifier circuit 213, preferably non-linear, the outputfrom which (FIG. 13i) controls the circuit 203 and 204 (this rectifierincludes in practice a filter circuit).

On the other hand the time discrimination is different from that as FIG.9. In the present case the output from the delay circuit 252 offrequency 2/τ controls another modulator by phase reversal 2320 whichreceives at its input the output from the first modulator 2120. Itsoutput is applied to another coherent integrating filter circuit 2321.Finally a multiplier 254 multiplies the outputs from the two coherentintegrating filters 2121 and 2321. Preferably the multiplier 254provides the non-linear characteristic suitable for favouring thestronger signals. One skilled in the art will understand that the outputfrom the multiplier 254 represents the time separations between thelocal timing-mark and the timing-mark received, just as thediscriminator 254 in FIG. 9 did. After that the output from the circuit254 feeds the integrating filter circuits 256 of the time tracking loopwhich in turn adjusts the delay θ in the delay circuit 252.

In FIG. 13f it appears that the channel A is giving a strong signal Swhilst the channel B is giving a disturbed signal of distinctly weakeramplitude, S/30, for example. This state of things is found again at theoutput from the filter 2121 (FIG. 13h). The rectified signal (13i),given by the rectifier circuit 213 is much stronger during the timingsegments associated with the channel A. Hence the automatic gain controlas in the other embodiments is sensitive above all to the undisturbedsignal from this channel.

As far as the time tracking is concerned, the output from the multiplier254 is connected with the function of correlation of the code receivedand the local code. This correlation function is represented in FIG.13j. It may be seen that it is much stronger for the channel A than forchannel B. One skilled in the art will understand that the time trackingloop then reacts essentially to the undisturbed signals from the channelA.

Thus the local pseudo-random code generator circuit 2511 contains thetiming mark received. It may be found again in the time base form T₁+T_(p) +K.T_(R) by carrying out a predetermined decoding of thedifferent stages of the shift register incorporated in the codegenerator 2511.

A local timing-mark may be defined in the same way: anotherpseudo-random code generator 261 like the first one 2511 is connected tothe output from the clock 251 across the divider of frequency by two262; the same decoding is carried out on the code generator 261 as onthe first one 2511, which gives the timing-mark local to the receiver,T₂ +P.T_(R). Finally, as previously, a clock 260 measures the separationbetween the two timing-marks. Of course numerous variants exists as tothe way of employing the timing-mark received as contained in the codegenerator 2511.

VI--EXAMPLES OF PREFERRED APPLICATIONS

The invention is capable of numerous applications already enumerated.Amongst these the measurement of propagation time and the measurement ofdistance are important. A number of examples will now be described byreference to FIGS. 14 to 16. These applications involve connections outand back between two points or redundant connections between more thantwo points, which enable the unknowns to be eliminated from the timebases (the instants T₁ and T₂).

FIG. 14 illustrates a connection out and back between an interrogator1000 and a responder 2000.

The interrogator 1000 includes a transmission time base 1113 thetiming-mark of which is T₁ +K.T_(R). It is connected to a transmissioncircuit 1110 which combines the functions of coding (112, FIG. 7) andamplification of transmission (111, FIG. 7). A duplexer 1001 couples theoutput from the transmission circuit 1110 to the sole transmissionantenna 1101. (It will be observed that by removing the thousands digitthe numerical references of components already described are often foundagain; in what follows they will not all be described in detail again).These circuits of the interrogator 1000 effect the transmission as forFIG. 7.

For its part the responder 2000 also includes circuits like those inFIG. 7. Two antennae 2201 and 2202 are connected alternately by a switch2205 to a duplexer 2001. The latter feeds in particular a receiving unitlike that in FIG. 7 (components 2203 and 2204, 2211 to 2213 and 2251 to2256).

Instead of being applied to a clock, the output 22523 from the circuit2252 feeds in its turn a transmitter unit 2110 (like 1110), if necessarythrough a circuit 2002 which defines a constant delay D. The transmitter2110 is also coupled to the duplexer 2001 in order to feed the antennae2201 and 2202 alternately through the switch 2205 controlled by thedivider 2206 from the time base 2251.

Thus the timing-mark received by the responder 2000 (output 22523 from2252) is sent by its transmitter 2110.

Between the latter and a receiving unit incorporated in the interrogator1000 the antennae 2201 and 2202 (now transmission) and the antenna 2101(now reception) define a space-diversity connection, this time as inFIG. 9.

Thus in the interrogator 1000 the duplexer 1001 connects the antenna toa receiving unit as in FIG. 9. The receiving unit includes the receivingchain 1211 to 1213, the automatic gain control loop 1203 and 1204, andthe time tracking loop 1252, 1254 and 1256. The timing-mark output 12523from the circuit 1252 provides the timing-mark initially transmitted,delayed by twice the propagation time T_(p), by the possible delay D dueto the circuit 2002 in the responder and small delays which are known,due to the circuits themselves.

A clock 1260 measures the time separation between the output 12523 andthe origin time base 1113. From it in known manner is deducted thepropagation time T_(p), with an ambiguity equal to the period T_(R) ofthis time base. This ambiguity may be removed at least partially bymeans of techniques which are likewise known.

In practice the transmission out between the interrogator 1000 and theresponder 2000 and the transmission back between the responder 2000 andthe interrogator 1000 are carried out over different high carrierfrequencies or else in shared time over the same carrier frequency underthe control of their respective time bases 1113 and 2251. In the lattercase a delay D sufficient for the response is employed by means of thecircuit 2002.

Such a connection out and back may be made general to a number ofresponders as shown in FIG. 15 where responders R1,R2,R3, . . . Rn maybe distinguished, built in the same way as the responders 2000 in FIG.14. In order to simplify the drawing only the two antennae and theswitch associated with each responder are illustrated. By T_(p1),T_(p2),T_(p3) . . . T_(pn) are designated the respective propagationtimes of the waves between the responders R1,R2,R3, . . . Rn and theinterrogator 1000.

For its part the interrogator formed as in FIG. 14 with, however,certain components repeated n times.

The time base 1113 is connected (connection not shown) to thetransmitter 1110 coupled to the antennae 1101 through the duplexer 1001.For reception the latter connects the antenna to the receiver amplifier1211 which is associated with its automatic gain control 1204.

On the other hand a register 1800 connected to the time base 1113produces n reception control signals separated in time, designated by X1to Xn; the signals X1 to Xn define n reception time spans assignedrespectively to the n responders for the interrogation and the responseof each of them. For example, between the instant O and the instantT_(R) the responders R1 is interrogated; from T_(R) to 2.T_(R) it is R2and so on until the last one Rn between the instant (n-1).T_(R) andn.T_(R). The cycle is then repeated periodically. Recognition by thevarious "responder" stations of the interrogations which areparticularly assigned to them may be realized by well known means ofsynchronization or coding.

Hence the "receiving" portion of the interrogator 1000 is sub-dividedafter the amplifier 1211 into n channels assigned respectively to the nresponders. Each channel includes the decoder 1212 followed by thenon-linear circuit 1213. The output from the latter is applied to theintegrating filter circuit 1203 as well as to the time discriminator1254 which forms the tracking loop with its integrating filters 1256 andthe controlled delay circuit 1252 connected to the time base 1113.Finally in each channel a switch such as 1801 fed by the signal X1 forthe channel 1 blocks the delay circuit 1252 outside the signal X1, thatis to say, outside the moments when the channel 1 is cooperating withthe responder R1.

All of the automatic gain control integrating filter circuits such as1203 are connected by a series of switches (reacting to the signals X1to Xn) to the input to the circuit 1204. Otherwise when it is notblocked (hence X1 . . . Xn in turn) each delay circuit such as 1252applies its output to the clock 1260 which compares it with the timebase 1113. As previously indicated the time separation measured eachtime by the clock is representative of the values 2.T_(p1),2.T_(p2), . .. 2.T_(pn), during the successive moments X1, . . . Xn. A final set ofswitches brings the values measured to a series of storage registerssuch as 1811.

The circuits which have just been described are capable of numerousvariants, in particular as to the realization of the multiplexedchannels associated with each of the responders.

The positions of the n responders being known, the interrogatordetermines the n transit times towards them, hence its distance withrespect to each responder and thereby its position. (Active circularradio-navigation).

FIG. 16 illustrates another example of application. The transmitter Eois that having two antennae as FIG. 9. Responders R1 to Rn likewisehaving two antennae respond to this transmitter each in turn as for FIG.15. Their signals are collected by a pure receiving station RP like theinterrogator 1000 in FIG. 15 but without the transmitter 1110 or,consequently, the duplexer 1001. The transmitter and responders arefixed (or have known positions). Hence the transit times of the wavesbetween Eo and R1,R2, . . . Rn are known, which are written C1,C2, . . .Cn.

The timing-mark of Eo is T_(o) +K.T_(R). The responder in row i, writtenR_(i), retransmits a timing-mark T_(o) +C_(i) +K'T_(R) (except for knownconstants).

The clock 1260 in the pure receiver RP will make time measurements withrespect to the local mark T₁ +K"T_(R) defined by the time base 1113.

The factors such as KT_(R) can be eliminated by known techniques forremoving ambiguity. The same applies for all of the known constant terms(C_(i) ; delays in the circuits). The responders themselves are usuallyarranged in order to correct the timing-mark which they retransmit inorder that it may be exactly T_(o) (+K'T_(R)).

Hence in the registers connected to the output from the clock 1260 willbe found T_(po) +T_(o) -T (register 1811), T_(p1) +T_(o) -T₁, T_(p2)+T_(o) -T₁, . . . T_(pn) +T_(o) -T₁. The receiver RP has not in generalavailable the timing mark T_(o) employed by the transmitter Eo and thefixed responder R1 . . . Rn. In this case, by forming the differencebetween the contents of the registers, T_(o) -T₁ disappears and oneobtains T_(po) -T_(p1), T_(po) -T_(p2) and all of the differences of thesame species. These differences in propagation time correspond withdifferences in distances of the receiver RP with respect to the stationsEo, R1, R2 . . . Rn. The receiver RP can determine its position byintersection of networks of hyperbolic position lines (hyperbolicradio-navigation).

The applications of the invention to measurements of distances and/orpropagation times are particularly advantageous, especially those whichhave just been described. The invention in short for the first timeenables terrestrial connections to be employed for this purpose overlong distances (transhorizon), which benefit from the accuracy allowedby waves of short wavelength (higher than 30 MHz or better 100 MHz, thatis to say, metric, decimetric, or centimetric waves). Decimetricwaves--frequencies of the order of 400 MHz--are at present preferred.

FIGS. 14 to 16 illustrate simple cases, non-restrictively. Thus thestations called fixed may, for example, be moved provided that theirpositions are known to the moving object which is determining itsposition (during or after the measurements). Any of the variants ofapplication mentioned after the first embodiment (Section IV) may bemade general with the examples as FIGS. 14 to 16.

The same applies for any of the variants upon coded transmissionpreviously described, especially in Section III as well as withreference to FIGS. 9 and 11 combined and 12 and 13 combined.

Thus, for example, instead of exploiting the installation as FIG. 16 inthe hyperbolic mode the position of the receiver RP may be determined bysolving the matrix equation of the n measurements contained in theregisters such as 1811 (determination of two-dimensional positioncoordinates by the method of "pseudo-distances"). This method yields asa supplementary result the separation between the original timing-markT_(o) (+KT_(R)) and the local timing-mark T₁ (+K".T_(R)), as soon asthere are at least 3 stations available. This time separation is knownwith an accuracy which depends upon the validity that may statisticallybe granted to each measurement. Knowing this time separation, thereceiver RP can then reconstitute a local timing mark which issynchronous with the original mark.

The present invention by enabling satisfactory transmission very muchbeyond the horizon, of a timing-mark, considerably enriches thepossibilities of measurement of propagation time and/or of distance, andmore generally the possibilities of terrestrial transmission(ground-to-ground) of time information.

What is claimed is:
 1. A system for the radio-electrical transmission ofa high-frequency signal carrying a timing-mark, comprising:(a)transmitter circuit means for generating a first high-frequencyelectrical signal carrying a basic timing-mark which is predeterminedand repetitive; (b) a first antenna station coupled to said transmittercircuit means for transmitting waves corresponding to said firsthigh-frequency electrical signal carrying said basic timing-mark; (c) asecond antenna station remote from said first antenna station, one ofsaid first and second antenna stations having at least two spaced-apartantennae so as to define at least two different propagation paths forhigh-frequency waves between said first and second antenna stations,said second antenna station comprising means for receiving saidtransmitted waves carrying said basic timing-mark after travel thereofover said at least two different propagation paths, and for deliveringcorresponding high-frequency electrical signals which carry receivedtiming-marks, said received timing-marks being the same as said basictiming-mark but being shifted in time with respect to said basictiming-mark in dependence upon the travelling time of the waves overeach of said different propagation paths; and (d) receiver circuit meanscoupled to said second antenna station for obtaining the receivedhigh-frequency electrical signals therefrom, said receiver circuit meanscomprising non-coherent combining means for(1) separately decoding, foreach of the different propagation paths, the corresponding receivedhigh-frequency electrical signals carrying the timing-marks which aredelivered by the second antenna station, (2) rectifying the separatelydecoded high-frequency electrical signals carrying the timing-marks inorder to recover said timing-marks, and (3) non-coherently combiningtogether the recovered said timing-marks into a compositetiming-mark,whereby said composite timing-mark is more closely relatedto the travelling time of the waves between said first and secondantenna stations than each of the recovered timing-marks individually.2. The system of claim 1, wherein said receiver circuit means is furtheroperative for weighting the recovered timing-marks according torespective weights related to the level of high-frequency signalreception over the propagation paths from which the recoveredtiming-marks arise, prior to said step of non-coherently combining therecovered timing-marks.
 3. The system of claim 2, wherein said receivercircuit means includesa circuit for variable-gain amplification of thereceived high-frequency electrical signals from the second antennastation, and gain control means for defining the variable gain of saidcircuit in accordance with the peak amplitude of the compositetiming-mark signal, whereby the recovered timing-marks are weighted as afunction of the level of the respective high-frequency signals fromwhich they have arisen, and thus according to the respective propagationpath travelled.
 4. The system of claim 3, wherein said gain controlmeans has a predetermined time constant which is greater than theaverage interval of time for self-correlation of the anomaliesobservable in transhorizon propagation.
 5. The system of claim 4,wherein said gain control means comprises an electrically non-linearmember located downstream of the variable-gain amplification circuit,the variable gain being dependent upon the output from said non-linearmember.
 6. The system of claim 1, wherein the transmitter circuit meanscomprises:a transmission amplifier circuit coupled to the first antennastation, a time base circuit, and a coder circuit connected to the timebase circuit for establishing from the time base the high-frequencysignal carrying said basic timing-mark, and applying the same to saidtransmission amplifier circuit.
 7. The system of claim 1, wherein thereceiver circuit means further includes:a receiving amplifier circuitcoupled to the second antenna station for amplifying the receivedhigh-frequency signals carrying the timing-marks, said non-coherentcombining means being coupled to an output of said receiving amplifiercircuit, a time base circuit for defining a local timing-mark, avariable delay circuit connected to the time base circuit for applying avariable delay to the local timing-mark, and a correcting circuit whichresponds to an output of the non-coherent combining means by controllingthe variable delay circuit such that the delayed local timing-markcoincides with the composite timing-mark, the delay so controlled beingrelated to the propagation time of the waves from the first antennastation to the second antenna station.
 8. The system of claim 7,whereinthe transmitter circuit means produces said first high-frequencyelectrical signal in the form of signal trains having a knowndistribution in time, the receiver circuit means receives said signaltrains delayed by the propagation time of the waves over saidpropagation paths, and the receiver circuit means includes means forauthorizing said decoding only during time intervals which have the samedistribution in time as the transmitted signal trains and which arelocated in time according to an output of said delay circuit.
 9. Thesystem of claim 7, wherein the correcting circuit comprisesa timediscriminator circuit coupled to receive the composite timing-mark fromthe non-coherent combining means and to receive the delayed localtiming-mark from the variable delay circuit, for providing an outputrepresenting a time difference between the composite timing-mark and thelocal timing-mark, and a time-constant circuit responsive to the timediscriminator circuit output for controlling the variable delay circuitin dependence upon said time difference.
 10. The system of claim 7,whereinthe transmitter circuit means produces short, high-frequency,high-energy pulses, the timing-mark being defined by said pulses, andthe receiver circuit means includes a switch for decoding said pulses,said switch being normally open and being closed during time intervalsdetermined by an output of the variable delay circuit.
 11. The system ofclaim 7, whereinthe transmitter circuit means produces signal trains ofhigh frequency modulated by a predetermined code, the timing-mark beingdefined by said code, and the receiver circuit means includes means fordecoding the received high-frequency signal trains during intervals oftime determined by an output of the delay circuit.
 12. The system ofclaim 11, wherein said predetermined code is a pulse compression code,and the decoding is effected by an operation of convolution orcorrelation.
 13. The system of claim 11, wherein said predetermined codeis a pseudo-random code, and the decoding is effected by an operation ofconvolution or correlation.
 14. The system of claim 1, wherein theantenna station having at least two antennae is said second antennastation, and wherein said receiver circuit means includes a respectivechannel to which each antenna of said second antenna station is coupled,each channel of said receiver circuit means comprising in series:areceiving amplifier circuit, and a timing-mark decoder circuit forming apart of said non-coherent combining means, the outputs of saidtiming-mark decoder circuits being combined to form the compositetiming-mark.
 15. The system of claim 1, wherein the antenna stationhaving at least two antennae is said second antenna station, theantennae of said second antenna station being coupled to the receiverconduit means, and wherein the receiver circuit means comprises:a singlereceiving amplifier, means for alternately connecting said antennae toan input of the receiving amplifier, and a timing-mark decoder circuitforming a part of said non-coherent combining means and having an inputcoupled to the receiving amplifier and an output which provides thecomposite timing-mark.
 16. The system of claim 1, wherein the antennastation having at least two antennae is said first antenna station, saidtransmitter circuit comprises means for alternately supplying said firsthigh-frequency signal carrying the basic timing-mark to said antennae,and said receiver circuit means comprises a single channel including:areceiving amplifier circuit, and a timing-mark decoder circuit forming apart of said non-coherent combining means and having an input coupled tothe receiving amplifier and an output which provides the compositetiming-mark.
 17. The system of claim 1, further comprising secondtransmitter means, coupled to said receiver circuit means and to saidsecond antenna station, for elaborating and supplying to said secondantenna station a second high-frequency electrical signal carrying asecond timing-mark, said second timing-mark being formed by saidcomposite timing-mark from said receiver circuit means, whereby saidsecond antenna station transmits and said first antenna station receiveswaves travelling over said at least two different propagation paths andcorresponding to said second high-frequency electrical signal carryingsaid second timing-mark.
 18. The system of claim 17, further comprisingsecond receiver circuit means coupled to said first-mentionedtransmitter circuit means and to said first antenna station forobtaining the received second high-frequency signals from said firstantenna station, said second receiver means comprising non-coherentcombining means for(1) separately decoding, for each of the differentpropagation paths, the corresponding received second high-frequencyelectrical signals carrying the second timing-marks which are deliveredby the first antenna station, (2) rectifying the separately decodedsecond high-frequency electrical signals in order to recover said secondtiming-marks, and (3) non-coherently combining together the recoveredsecond timing-marks into a second composite timing-mark,whereby thephase shift between said second composite timing-mark and said basictiming-mark represents twice the propagation time of the waves betweenthe first and second antenna stations.
 19. The system of claim 17,further comprising:(a) a third antenna station remote from and operativefor recovering the waves transmitted from said first and secondtransmitter stations, the third antenna station receiving waves fromsaid first antenna station over at least two different propagation pathsand receiving waves from said second antenna station over at least twopropagation paths, and (b) second receiver circuit means coupled to saidthird antenna station for obtaining received high-frequency signalscarrying timing-marks therefrom, said second receiver circuit meanscomprising non-coherent combining means for(1) separately decoding, foreach of the different propagation paths to said third antenna station,the high-frequency signals delivered from the third antenna station andcarrying timing-marks, (2) rectifying the separately decodedhigh-frequency electrical signals delivered from the third antennastation and carrying timing-marks in order to recover the timing-markstherefrom, (3) non-coherently combining together the timing-marksrecovered from waves received from said first antenna station into asecond composite timing-mark, and (4) non-coherently combining togetherthe timing-marks recovered from the waves received from said secondantenna station into a third composite timing-mark,whereby the timeseparation between said second and third composite timing-marks isrelated to the difference in distance between the third antenna stationand the first and second antenna stations, respectively.
 20. The systemof claim 1, wherein the distance between said two-spaced apart antennaeis between about 15 and about 30 wavelengths of the waves correspondingto said first high-frequency signal.
 21. The system of claim 1, whereinsaid transmitted waves corresponding to said first high-frequency signalhave a frequency higher than about 30 MHz, preferably higher than about100 MHz.
 22. The system of claim 1, wherein said transmitted wavescorresponding to said first high-frequency signal have a frequencybetween about 300 MHz and about 3000 MHz.